This invention relates generally to disposable and refillable inkjet printing cartridges, and more particularly to a bubble generator and vent cover for an inkjet cartridge and a method for filling an inkjet cartridge.
Inkjet printing generally involves the controlled delivery of ink drops from an inkjet cartridge reservoir to a printing surface. One type of inkjet printing, known as drop-on-demand printing, employs a pen that has a print head responsive to control signals for ejecting ink drops from the reservoir. Drop-on -demand type printheads typically use one of two mechanisms for ejecting drops: thermal bubble or piezoelectric pressure wave. A thermal bubble type printhead includes a thin-film resistor that is heated to cause sudden evaporization of a small portion of ink within an inkjet nozzle. The rapid expansion of the ink vapor forces a small amount of ink through the nozzle's orifice. Piezoelectric pressure wave type printheads use a piezoelectric element responsive to a control signal for abruptly compressing a volume of ink within a nozzle. A resulting pressure wave forces the ink through the nozzle orifice.
The drop-on-demand printhead is effective for ejecting or "pumping" ink drops from a cartridge's ink reservoir. When the printhead is inactive, there is need to prevent ink from permeating (or leaking) through the printhead. To prevent such occurrence it is preferred to create a slight back pressure at the printhead when inactive to prevent leakage. The term "back pressure", as used herein, means the partial vacuum within an inkjet reservoir that resists flow of ink through the printhead. Back pressure is considered in the positive sense so that an increase in back pressure represents an increase in partial vacuum. Accordingly, back pressure is measured in positive terms, such as water column height.
It is desired that the back pressure at the printhead be at all times strong enough to prevent ink leakage. The back pressure, however, is not to be so strong that the printhead is unable to overcome the back pressure to eject ink drops. Moreover, it is desired that the inkjet cartridge be designed to operate despite environment changes causing fluctuations in back pressure.
A severe environmental change that affects reservoir back pressure occurs during air transport of an inkjet cartridge. In such instance, ambient air pressure decreases as an aircraft gains altitude and is depressurized. As ambient air pressure decreases, the pressure differential between the environment and the reservoir decreases. To maintain the pressure differential, back pressure needs to be increased (i.e., make more negative) to keep ink from leaking through the printhead. Accordingly, the level of back pressure is regulated during times of ambient pressure drop.
Changes in back pressure also occur during operation. As the printhead ejects an ink drop, the depletion of ink from the reservoir increases the reservoir back pressure (i.e., makes more negative; creates greater vacuum). Without regulation of the back pressure, the inkjet cartridge eventually will fail prior to using all the ink in the reservoir. Specifically, the inkjet printhead is unable to overcome the increased back pressure to eject ink drops.
Mechanisms for regulating inkjet reservoir back pressure dynamics in response to environmental changes and operational effects include mechanisms commonly referred to as accumulators. The accumulators are designed to move between a minimum volume position and a maximum volume position in response to changes in the level of the back pressure within the reservoir. Accumulator movement changes the overall volume of the reservoir to regulate the back pressure to maintain back pressure within desired operating range.
For example, the accumulator increases reservoir volume as the difference between ambient pressure and back pressure decreases. This increases back pressure sufficiently to avoid leakage. In effect the increase in back pressure caused by the accumulator counters the decrease in the difference between ambient pressure and back pressure. Thus, a reliable difference between ambient pressure and back pressure is maintained to avoid leakage.
The accumulator decreases reservoir volume when environmental changes or operational effects (e.g., ink depletion) cause an increase in back pressure. The decreased volume decreases back pressure sufficiently to avoid printhead failure. In effect the decrease in back pressure caused by the accumulator counters the increase in the difference between ambient pressure and back pressure. Thus, a reliable difference between ambient pressure and back pressure is maintained to avoid printhead failure.
Accumulators have been used in conjunction with devices referred to as bubble generators. A conventional bubble generator permits ambient air bubbles to enter the ink reservoir once the accumulator has moved to its maximum volume position (that is, once the accumulator is unable to further reduce back pressure) and back pressure continues to increase. Air bubbles delivered by the bubble generator increase the air volume in the ink reservoir in an effort to prevent back pressure from reaching a level causing printhead failure. Bubble generators generally include an air inlet passage and a small-diameter orifice that provides communication between the bubble generator and ambient air. The reservoir back pressure maintained by the accumulator prevents ink from leaking through the bubble generator orifice. With the reservoir being positioned over the bubble generator orifice, ambient air is unable to enter the reservoir and alter the back pressure until back pressure increases enough to draw an air bubble through the orifice into the reservoir.
One problem addressed by the invention is that variations in inkjet cartridge part dimension due to manufacturing tolerances or changes in ambient temperature cause variations in bubble pressure at the bubble generator. Such variations in bubble pressure in turn affect print quality and inkjet cartridge performance. Accordingly, there is need for a bubble generator which produces more consistent bubble pressures across changes in ambient temperature and pressure.
Another problem addressed by this invention is related to the welding of a vent cover over the bubble generator. Conventional inkjet cartridges include a welded vent cover having an opening leading to the bubble generator. Flash or very small pockets in the welding have been observed to occur between the cover and the area of the cartridge being covered. During specific pressure changes, ink is suspected of wicking through the bubble generator into the flash and pockets. Eventually the ink dries into a porous patch. Such porous patch is more prone to drawing ink during pressure changes and can result in less effective vent inlet performance for the bubble generator. As a result, there is a need for a more reliable method for attaching the vent cover.
Additional problems addressed by this invention relate to the difficulty in priming an inkjet cartridge and the susceptibility toward depriming of the inkjet cartridge. A conventional inkjet cartridge 10 (see FIG. 1) includes a casing 12 with an internal ink reservoir 13. For purposes of naming consistency a bottom surface 14 of the casing refers to a bottom surface in the cartridge's installed ready-to-print position. A printhead 16 typically is at a most bottom portion of the case. The bubble generator 18 also is located along a bottom surface of the case. A vent cover 19 covers the bubble generator. The accumulator 20 is located within the reservoir. There is an opening 21 in a top surface of the case proving a vent to accumulator 20 air bags. Typically there is another opening 22 in the top surface of the case through which ink is poured to fill the reservoir. A plug 24 blocks the opening once the ink is loaded. The opening 22 remains plugged during operation. Once the ink is loaded and the opening is plugged, the cartridge is to be primed. Priming is performed to remove air from the printhead region and to initially draw the ink into the printhead nozzles. Priming is achieved by sucking air through the printhead nozzles thereby drawing ink from the reservoir into the printhead.
Prior to ink fill, a piece of tape is placed over the bubble generator vent to prevent leakage during filling. Once filled and after priming, another piece of tape is placed over the nozzles. The two pieces of tape prevent drooling and clogging of the openings during shipment. Upon shipment an end user removes the tapes allowing operation of the cartridge. Failure to remove the nozzle tape precludes ink ejection and is quickly remedied by the user. Failure to remove the bubble generator tape, however, has no immediate effect. Thus, it is less evident to an end user that the cartridge has been incorrectly placed into service. Specifically, if the tape over the bubble generator vent is not removed, the cartridge eventually will deprime before most of the ink is consumed. Depriming is caused by lack of back pressure regulation. Once enough ink has been consumed that the accumulator is no longer able to regulate back pressure and the bubble generator is supposed take over, failure of the bubble generator to take over causes an increase in back pressure and eventually air intake and depriming through the nozzle orifices. Once deprimed, the cartridge ceases to eject ink droplets even though there is still a substantial amount of ink in the reservoir. Typically, an end user does not have the equipment to reprime the cartridge. Thus, for the disposable cartridge the useful life of the cartridge is substantially shortened by a user's failure to remove the tape over the bubble generator vent. Accordingly there is need for an improved fail-safe to avoid depriming.